1. Field of the Invention
The present invention relates to a method for extending the life of dehydrogenation catalysts. The present invention particularly relates to a method for extending the life of catalysts used to dehydrogenate alkyl aromatic hydrocarbons to form vinyl aromatic hydrocarbons.
2. Background of the Art
Vinyl benzenes play a particularly important role in the preparation of synthetic plastics and resins. The polymerization of styrenes, for example, to produce polystyrene resins is well known.
Styrene and styrene derivatives are typically produced from ethylbenzene materials by dehydrogenation over solid catalysts in the presence of steam. Catalysts found to be effective and useful for this process include those based on potassium oxide promoted, chromium oxide stabilized, iron oxide materials as disclosed in, for example, U.S. Pat. No. 5,503,163 to Chu. Another catalyst reported to be useful for preparing styrene from ethylbenzene is itself prepared from extruded alpha-FeOOH iron oxide particles derived from scrap metal via dehydration of a yellow alpha-Fe(OOH) intermediate via the Penniman process and devoid of bound sulfate, having a median size of at least 2 microns. These catalysts, as disclosed in U.S. Pat. No. 5,689,023 to Hamilton, Jr., also are prepared from a formulation that includes a potassium containing compound. U.S. Pat. No. 6,184,174 B1 to Rubini, et al., discloses catalysts for dehydrogenating ethylbenzene to styrene which comprise iron oxide, oxides of alkaline and alkaline-earth metals, oxides of the lanthanide series, oxides of metals of the sixth group, prepared from a paste of iron oxide impregnated with an aqueous solution of a cerium salt, treated with aqueous KOH and calcined, after drying, to pre-form potassium ferrate, to which the other components or precursors of the catalyst are then added.
A typical process for preparing styrene is the so-called “Dow Process.” As reported in “The Chemical Engineers' Resource Page” at http://www.cheresources.com/polystymonzz.shtml, in the DOW Process, the energy needed for the conversion of ethylbenzene to styrene is supplied by superheated steam at about 720° C. that is injected into a vertically mounted fixed bed catalytic reactor with vaporized ethylbenzene. The catalyst is iron oxide based and contains Cr2O3 and a potassium compound, KOH or K2CO3, which act as reaction promoters. Typically, 0.7-2.0 kg of steam is required for each kilogram of ethylbenzene to ensure sufficiently high temperatures throughout the reactor. The superheated steam supplies the necessary reaction temperature of 550-670° C. throughout the reactor.
After the conversion reaction, the products are cooled rapidly to prevent polymerization. The product stream, containing styrene, toluene, benzene, and unreacted ethylbenzene, is fractionally condensed after the hydrogen is flashed from the stream. After adding a polymerization inhibitor, the styrene is vacuum distilled in a series of four columns, often times packed columns, to 99.8% purity. Typical capacity per plant ranges from 200,000 to 400,000 metric tons per year in each reactor and most plants contain multiple reactors or units.
While processes such as the Dow Process are useful for preparing commercial quantities of styrene and other vinyl aromatic hydrocarbons, these process are not without problems. One major problem is loss of catalyst activity. As with most reactions using catalysts wherein the catalyst is retained within the process, the catalyst may, with time, lose reactivity and selectivity. Simply replacing aged catalysts is usually not an economically viable resolution of this problem. The above-described catalysts are expensive to make and replacing the catalyst usually requires that the operating unit be shut down. Any loss of production time is always undesirable and excessive maintenance can greatly increase the cost of producing vinyl aromatic hydrocarbons, such as styrene. It would be desirable in the art of producing vinyl aromatic hydrocarbons to extend the life of catalysts used in vinyl aromatic hydrocarbon production.